refs https://github.com/TryGhost/Ghost/pull/11152
- Added subscribers table drop migration
- Removed subscribers from schema
- Removed subscribers controllers/routes/regression tests
- Removed subscriber related API code
- Removed subscribers from internal apps
- Removed subscriber importer
- Removed subscriber model
- Removed subscriber related permissions
- Removed webhook code related to subscribers
- When upgrading to v3 it is on the site admin to migrate all zapps or any other webhook clients to use members
- Removed subscriber-specific translation
- Removed subscriber lab flag